/**
* @author AlbertDarren
* @contact 2563491540@qq.com
*/
#include <cstdio>

using namespace std;

int main_29()
{
    int n,sum=0;
    scanf("%d",&n);
    for (int i=1;i<=n ;++i )
    {
        if (sum>2000)
        {
            break;
        }
        sum+=i;
    }
    printf("%d\n",sum);
	return 0;
}
